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  1. #1
    Join Date
    Nov 2008
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    Perth WA
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    71
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    Default Scraping and Struggling.

    You blokes are going to come down of me like a ton of bricks....

    I am trying to create a flat surface on a piece of cast iron measuring 130 x 80mm. It is clamped to the mill table because there are more milling operations involved and more importantly, there is 0.001 " fall in the table on the X axis over the 130mm length. I did not check before I started cutting.
    This is not a job for the surface plate that have. It is cast iron, heavy and not something that lends itself to delicate positioning on the workpiece.

    I started off using a lathe faceplate, then a Moore and Wright angle plate and finally a surface ground magnetic chuck as a portable surface plate. I can see the heads shaking.

    I've lost the lid off the bearing blue tube and as a result there is blue from A to B.

    Traversing an indicator along the X axis reveals nary a flutter of needle movement but there is 0.0005" fall towards the front edge in the Y. I have to keep going.

    A couple of real basic questions. Is bearing blue the stuff I should be using and even when very thinly spread on my "surface plate", it smears somewhat on the workpiece when the plate is rubbed on the surface, is this what usually happens?

    Bob the tyro.

  2. #2
    Join Date
    May 2010
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    Lower Lakes SA
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    58
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    Default

    Bob, some bearing blues work better than others. I have some of that powerplus and another which I like better. I forget exactly why now. I will check the brand tomorrow (it's late here). Smearing will happen on a ground surface. Spotting masters always have a little texture, whether from scraping on an iron tool, or the the natural texture of the stone with granite.

  3. #3
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    Nov 2008
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    Default

    Thank you Bryan.

  4. #4
    Join Date
    Jul 2010
    Location
    Melbourne
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    Default

    Hi BT,
    I'm confused. Why cant you take the part the surface plate?(I assume because you don't want to reset?)
    The problem with what you are doing(if I understand you) is you will end up with a flat face but it wont be parallel to the bottom face(or are you going to come back and fix that up later?)
    I found spotting off surface ground surfaces did smear easily. Are you getting consistent spotting with the magnetic chuck?

    Certainly looks nice.

    Stuart

  5. #5
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    Default

    Hello Stu,

    I didn't want to remove the workpiece from the table because I have to bevel the long edges and then bore a center hole and cut a tee slot. Leaving it in place assures parallelism. Let's say my adjustment of the table on the X axis is a shabby as it is on the Y, the chances of repositioning the workpiece precisely are less than slim, if I removed it for more convenient access.

    The bottom being out of whack is of no account. It will be machined true when I turn the dovetail spigot. The supposedly flat scrapped surface will be bolted to the lathe faceplate.

    Given the slipperyness of the surface ground mag chuck, do you consider it better to use the flycut surface of the angle plate as my surface plate?

    BT

  6. #6
    Join Date
    Oct 2011
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    Norwood-ish, Adelaide
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    59
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    6,541

    Default

    Bob, the amount that you are taking off by scraping is in the order of a thou or two. On that basis doing the machining operations and then removing the part to scrape should not be an issue.
    Connelly sometimes advocates using one part of a machine as a master for another part that is being scraped in. On that basis, the best thing to use may be the mating part.

    Michael

  7. #7
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    May 2010
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    Default

    Quote Originally Posted by Michael G View Post
    Connelly sometimes advocates using one part of a machine as a master for another part that is being scraped in. On that basis, the best thing to use may be the mating part.

    Michael
    Yes, but after scraping the first part to a known reference I think.

    Bob, the better bearing blue I have is Holt's. But I think that's the least of your worries. You could use your faceplate or angle plate, after first scraping them - in theory to reference standard - using your surface plate. But if your surface plate is cast iron and second hand it can't really be trusted unless you've checked it against a known reference. The other problem is what are you running your indicator on? Obviously the mill table. But how do you know how flat that is? I don't mean to be discouraging, but unless you start with something known to be flat (enough), you could waste a lot of time.

    I don't really understand your problem with the mill setup but I'll leave that for others with more milling experience.

  8. #8
    Join Date
    Aug 2008
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    near Rockhampton
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    Default

    Quote Originally Posted by Michael G View Post
    B
    Connelly sometimes advocates using one part of a machine as a master for another part that is being scraped in. On that basis, the best thing to use may be the mating part.

    Michael
    But that is only when the other part has been scraped first.. In fact you always use the mating part as a master when doing final work, like fitting a tailstock or saddle to the ways..
    Gold, the colour of choice for the discerning person.

  9. #9
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    Default

    Bryan, Stu, Michael and Richard,

    Thank you for the advice. Given that nothing I can use as a master can be guaranteed as flat I'm sorely tempted to mount up the 28mm slot drill in the horizontal spindle and remove yesterday's handiwork. I can't use the mating base of the dividing head as a master because of the projecting bevelled edges. Bit of a catch 22.

    The universal table requires realignment if it is moved from zero on any axis. A fixed table would be a handy thing to have.

    BT

  10. #10
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    Oct 2011
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    Default

    How about a trip to a glazier and getting a small piece of the thickest plate glass they have? That stuff is usually pretty flat and as a first approximation will give you a reference.

    Michael

  11. #11
    Join Date
    Mar 2011
    Location
    Dural NSW
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    Default Flat Surfaces

    Bob
    I just happened to look up the forum & discover your current problem
    I agree with Michael, its going to be difficult to scrape a surface flat to a ground surface however its a start. I just checked my hand scraped surface plates one measures 7" by 9" & the other 9"by 12" the smaller one would be ideal for your job.
    I only wish you were a bit closer as I would drop over this afternoon.But its about a 5 day drive, I remember it last year, crossing the Nullabor.
    I use the Power Plus bearing blue & its ok providing you only use a little on the tip of the finger, too much & it just gets confusing & a mess.
    There must be an old Tooly in your area that could loan you a reference scraped surface to use. It would not take long to bring a surface that size to flatness.
    The other thought I had, was I have a roll of half thou .0005" thick paper (very thin) about 1/2" wide left over from my Tooling days. After you got the surface flat by scraping, the underside could be shimmed up with the half thou paper to set the horizontal surface clocked in at zero, for subsequent machining of your dovetail.
    I am not sure if this all will help but if it is of assistance I can post over some of the half thou paper to you.
    Once you start chasing these half thous ,the whole game gets interesting, to say the least.
    regards
    Bruce
    ps Now to get back to a wooden stair case, I have been building for the last 35 hours.
    Looking forward to its completion, & getting back to metal stuff, & dial gauges.

  12. #12
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    Jul 2010
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    Default

    Quote Originally Posted by Anorak Bob View Post
    It will be machined true when I turn the dovetail spigot.
    I'd forgotten about that part . I should have gone back the the drawing.


    Quote Originally Posted by .RC. View Post
    But that is only when the other part has been scraped first.
    If BT was making a way Yes. But I dont think the parts are going to move in use. So if the parts are going to be clamped together in the same orientation Do they need to be flat to a master?

    BT,
    Why not machine the Tee slot(+the relief/rebat what ever you want to call it) and bevelled edges now and use your dividing head as a master?


    Stuart(likely missing something as normal)


    p.s. you might want to spot your dividing head to the other table you have to be sure they fit well.

  13. #13
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    Jun 2008
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    Default

    Hi BT,

    I think the 2-4-6 blocks we did on the scraping course would do the trick as a reference surface, I think they should be about the right size for your job. Interested?

    Regards
    Ray

  14. #14
    Join Date
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    Sydney
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    Default

    Sorry Bob, I'm completely confused, normal I know

    Why can't you finish the milling operations and then scrape that surface from the surface plate? You can still confirm parallelism by indicating off the surface plate. I'm sure I've missed something here, but can't quite see what it is.

    Pete

  15. #15
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    Default

    What's causing me the most angst are the two bevels. They are critical because they provide the alignment of the head on the base. If I was to accurately machine them prior to scraping the flat face, any removal of material from that flat face is going to result in an increase in width between the bevels. The head will no longer fit. If I was to increase that width by say 0.0005" and then rely on scraping the bevel to ensure a correct fit I am all of a sudden out of my depth because I would have to scrape an edge about 10mm wide and 130mm long at 60 degress, uniformly.

    Stu is correct. The dividing head is clamped on this base in one fixed position.

    BT.

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